Low temperature curing corrosion resistant polyurethane coatings

ABSTRACT

A low temperature curing polyurethane coating system is provided which enables the deposition of adherent corrosion resistant coatings having a desirable balance of hardness and flexibility by using a mixture of a solution copolymer of from 55-85% styrene with the balance of the copolymer consisting essentially of monoethylenically unsaturated hydroxy functional monomer, the copolymer being in solution in inert organic solvent, and an organic polyisocyanate. The polyisocyanate is kept separate from the copolymer solution until the desired liquid mixture is formed and used.

The present invention relates to low temperature curing two-package polyurethane coatings adapted to deposit hard, flexible and corrosion resistant adherent films on steel surfaces. The invention includes the pretreatment of the steel to maximize adhesion.

It is known to provide two component coatings in which an organic polyisocyanate, on the one hand, is combined with an hydroxyl bearing resin, on the other hand. The mixture reacts at low temperature and can be applied before the reaction has proceeded to thicken the mixture excessively. The hydroxyl bearing resin is usually a polyester or polyether. The reaction produces a polyurethane, and it has the advantage that very little energy is required.

While these two component coating systems have many favorable attributes, they have not been well received by the appliance industry because adhesion to the steel substrate has been inadequate and the high corrosion resistance demanded by this industry has not been present. Corrosion resistance is used herein generically to embrace resistance to chemical attack which may be evidenced by inadequate resistance to detergents, salt spray, humidity, or staining.

As a result, and to provide coatings having the required resistance to chemical attack, the appliance industry has employed two coat systems, involving the application and separate baking at temperatures of at least about 300°-350°F. of a primer coat and a top coat. In the actual practice with which we are most familiar, the primer is baked 20 minutes in a 425°F. oven, and the topcoat is baked 20 minutes in an oven maintained at a temperature of from 350°-400°F. depending on the topcoat used. This requires separate coating lines, each having its own lengthy oven maintained at the high temperatures referred to.

Thus, the art has faced a dilemna. The properties needed have been available, but the cost has been high, and so has the curing energy requirement. On the other hand, the low energy curing one package polyurethane systems have not provided the needed properties.

The art has also been aware of the properties which normally flow from the selection of particular monomers. Thus, styrene has been avoided in thermosetting coatings which must be subjected to the elements, methyl methacrylate being chosen instead because of its superior properties, and this despite the fact that for many years styrene has been more available, and at lower cost. It is of interest to note that this availability and cost relationship do not prevail at the present time. Moreover, and regardless of which of these monomers is selected, the polymers made from them were unduly brittle unless a flexibilizing monomer was employed to provide an internal plasticizing action.

Curiously, in this invention, styrene is required to be used in large amount, and its replacement by methyl methacrylate or the presence of any significant amount of flexibilizing monomer in the copolymer function to degrade the high level of corrosion resistance which is essential.

In accordance with this invention, a solution copolymer is employed which contains from 55-85% of styrene, preferably from 65-80% of styrene, and the balance of the copolymer consists essentially of monoethylenically unsaturated hydroxy functional monomer. This copolymer, in organic solvent solution, constitutes one package of a two package system, the second package of which is an organic polyisocyanate.

It has been discovered that the use of these styrenebased hydroxy functional solution copolymers in the low temperature curing polyurethane two package systems described, unexpectedly yields excellent detergent, salt spray, and stain resistance. Moreover, the cure is rapid, and the cured product is hard and flexible, and has satisfactory impact resistance despite the very high level of hard and brittle styrene monomer, and the absence of any flexibilizing monomer. These well balanced properties in a low temperature curing system represent a significant advance in the art.

Replacement of styrene with methyl methacrylate causes a dramatic reduction in corrosion, detergent, salt spray, and stain resistance properties. Thus, the system which the art would think to be the better one, is unexpectedly markedly inferior and unsatisfactory for use on refrigerators, and laundry equipment.

The organic polyisocyanates are useful in this invention as a class, it being preferred to employ at least a stoichiometric proportion thereof, most preferably a 5-40% stoichiometric excess. Diisocyanates and polyisocyanates of higher functionality are fully useful, the latter being preferred. The conventional toluene diisocyanates will illustrate the most common useful material, but aliphatic polyisocyanates, such as hexamethylene diisocyanate, are preferred. Higher molecular weight polyisocyanates having an isocyanate functionality of at least 3 are particularly preferred, such as adducts of diisocyanates with polyhydric alcohols, such as trimethylol propane. Particularly preferred is the reaction product of three moles of hexamethylene diisocyanate with one mole of water, which is a triisocyanate having the formula: ##EQU1##

These polyisocyanates and also the solution copolymers are used in solvent solution, and this means that the solvent must be free of active hydrogen and free of water, as is well known.

Thus, the final mixture is a three component mix containing organic solvent, polyisocyanate, and solution copolymer. Other conventional materials may also be present, such as flow control agents, catalysts to speed the cure, pigments, fillers, soluble dyes, and the like, as is conventional.

Referring more particularly to the hydroxy functional copolymers which are used, these are solution copolymers which identifies copolymerization in organic solvent medium which produces an organic solvent soluble resin. The styrene component is critical as previously described. The hydroxy monomer is preferably an hydroxy alkyl acrylate, the alkyl group containing from 2-4 carbon atoms. Hydroxy ethyl acrylate is particularly preferred, and will be illustrated. It is noted in passing that the term hydroxyl is used in its conventional narrow connotation in which it identifies the alcoholic hydroxyl group, and no other group. Indeed, the hydroxyl group and the single ethylenic group are the only reactive groups contemplated in this monomer.

Other hydroxy monomers which may be used are simple adducts of an alkylene oxide containing 2-4 carbon atoms with an alpha, beta-monoethylenically unsaturated carboxylic acid. The oxides are illustrated by ethylene oxide, 1,2-propylene oxide, and 1,2-butylene oxide. The acids are illustrated by acrylic acid, methacrylic acid, crotonic acid, itaconic acid, maleic acid, etc. Thus, hydroxy propyl methacrylate, hydroxy ethyl crotonate, and hydroxy butyl maleate (the mono- or bis-ester) will further illustrate the class of useful monomers.

The solution copolymerization is wholly conventional, being normally carried out at reflux in the selected solvent in the presence of a free radical polymerization catalyst, such as benzoyl peroxide, t-butyl perbenzoate, or azobisisobutyronitrile.

The invention is illustrated in the following examples.

Example 1

Charge 400 parts 2-ethoxy ethanol acetate and 400 parts xylol to reactor and heat to 130°C. Premix 1070 parts styrene, 340 parts hydroxy ethyl acrylate, and 20 parts t-butyl perbenzoate and add to reactor over a 3 hour period, while maintaining reactor at 130°C. Hold 1 hour and add 4 parts t-butyl perbenzoate. Hold 2 hours and add 200 parts 2-ethoxy ethanol acetate and 200 parts xylol. The hydroxyl number of the product was 125.

The resin of Example 1 was evaluated in a pigmented low energy curing coating as follows:

    Copolymer Solution Component (pigmented) - Parts by Weight                     ______________________________________                                         14.47  Solution of Example 1                                                   5.36   2-Ethoxy ethanol acetate                                                32.15  Titanium dioxide                                                               Grind pigment dispersion, then add:                                     31.48  Solution of Example 1                                                   13.98  Butyl acetate                                                           2.14   Dibutyl tin dilaurate catalyst                                                 (1% solution in methyl ethyl ketone)                                    0.43   Silicone solution flow and anticratering agent                                 (optional - see Note 1)                                                        Solids by Weight 57.4%                                                         Weight per gallon 10.75                                                 ______________________________________                                         Isocyanate Solution Component                                                  Parts by Weight                                                                53.28  75% solution of adduct of 3 moles hexamethylene                                diisocyanate with one mole of water in a 1:1 weight                            ratio mixture of 2-ethoxy ethanol acetate and                                  xylol (see note 2)                                                      38.29  2-Ethoxy ethanol acetate                                                8.43   Toluene                                                                 ______________________________________                                          Note 1--Dow Corning 230 fluid having a viscosity at 77°F. of 1400       centistokes and an open cup flash point of 500°F. may be used.          Note 2--Desmodur N-75, a product of Mobay Chemical Company may be used.        Percent solids by weight: 40.0%                                          

For application by electrostatic spray, the following ratio by volume of the separate components is used:

63.83 Copolymer Solution Component

9.41 Toluene (added to reduce viscosity for handling)

26.76 Isocyanate Solution Component

Upon admixture, the two components form a solution which thickens gradually with time, but an adequate working period is available for hand spray. For electrostatic spray, the components are mixed on the way to the spray head. The mixture at a viscosity of 22 seconds measured at 65°F in a °2 Zahn Cup is pumped at 400 cc per minute to an electrostatic disc applicator charged to 90 kilovolts and spinning at 3,600 revolutions per minute. Electrostatic spray was carried out on grounded steel panels moving at 35 feet per minute past the spray disc at a distance of 13 inches. Atomization was excellent, and the deposited wet film showed no tendency to crater, bubble, or sag. The coated panels were baked briefly at low temperature (10 minutes in a 150°F. oven). The final film had a very good appearance, and a thickness of 1.0 mil. The cured panels were aged for 168 hours at room temperature during which period the coatings hardened from an F pencil hardness to a 2 H hardness. The 60° gloss meter reading was 98 and the coatings were adequately flexible as indicated by passing the 1/8 inch conical mandrel test, and exhibiting a direct impact in excess of 40 inch pounds. The coatings were strongly adherent to the base, and satisfactorily abrasion resistant. Also, stain resistance and grease resistance were satisfactory.

Various panels were coated, and some of these provided superior properties, particularly in the areas of impact resistance and flexibility, on the other hand. Accordingly, and for most outstanding results in these categories, the steel should be prerinsed with a rinse solution containing trivalent chromium. It is only in this way that we have been able to pass 240 hours detergent testing in 1.0% synthetic detergent, and 500 hours in 5% salt spray.

In this invention, the steel substrate is cleaned with detergent and rinsed with water, and is then phosphated in conventional fashion with an aqueous solution of zinc or iron phosphate.* After rinsing with water, a chromate solution is applied. This solution is aqueous, and it contains trivalent chromium, either alone or together with hexavalent chromium. Parcolene 8 supplied by the Parker Company of Detroit, Michigan, may be used.

To more precisely identify the new contribution which has been made, there is presented a Table I which summarizes the manner in which composition changes are related to physical properties and corrosion resistance. Only the styrene-hydroxyethyl acrylate copolymer passes the severe corrosion and flexibility tests of the appliance industry, when utilized in a single coat (no primer) finish.

                                      TABLE I                                      __________________________________________________________________________                      Styrene                                                                              Acrylic                                                                              Styrene                                                                              Commerical                                                   Polymer                                                                              Polymer                                                                              Acrylic                                                                              Polyester*                                                               Polymer                                           __________________________________________________________________________     Styrene           76%         38%                                              Hydroxy Ethyl Acrylate                                                                           24%   25%   25%                                              Ethyl Acrylate                38%                                              Methyl Methacrylate     76%                                                    Polyester (Hydroxy Functional)      100%                                       Humidity, 2000 hours                                                                            Pass  Fail  Fail  Fail                                        Detergent Resistance (hours)                                                                    Pass 240                                                                             Fail 48                                                                              Fail 48                                                                              Fail 24                                     5% Salt Spray Resistance                                                       (hours)          Pass 500                                                                             Fail 250                                                                             Fail 250                                                                             Fail 150                                    Flexibility (1/4" Mandrel)                                                                      Pass  Fail  Pass  Pass                                        Impact (inch/lbs.)                                                                              Pass 40                                                                              Fail 40                                                                              Pass 40                                                                              Pass 40                                     __________________________________________________________________________      *Mobay Chemical Company Polyester Desmophen-650                          

All of the above coatings were cured by baking 10 minutes at 150°F. followed by 168 hours of ageing at room temperature.

It will thus be evident that the liquid mixtures formed in this invention can be cured without exceeding 250°F., preferably without exceeding 200°F. Indeed, as little as 10 minutes in a 150°F. oven is adequate when followed by room temperature ageing for at least a day. With care, simple storage at room temperature will supply an adequate cure, though some heating is helpful to speed the development of a solid surface which is easier to handle in storage.

The invention is defined in the claims which follow. 

We claim:
 1. A low temperature curing polyurethane coating system comprising a liquid mixture of a solution copolymer of from 55-85% of styrene with the balance of the copolymer consisting essentially of monoethylenically unsaturated hydroxy functional monomer, said copolymer being in solution in inert water-free organic solvent, and an organic polyisocyanate in at least a stoichiometric proportion with respect to the hydroxy functionality in said copolymer.
 2. A coating system as recited in claim 1 in which said polyisocyanate is present in a stoichiometric excess of 5-40%.
 3. A coating system as recited in claim 2 in which said polyisocyanate is an aliphatic polyisocyanate having an isocyanate functionality of at least
 3. 4. A coating system as recited in claim 3 in which said polyisocyanate is an adduct of an aliphatic diisocyanate with a polyhydric alcohol containing at least three hydroxy groups.
 5. A coating system as recited in claim 3 in which said polyisocyanate has the formula: ##EQU2##
 6. A coating system as recited in claim 1 in which said hydroxy functional monomer is an hydroxy alkyl acrylate, the alkyl group containing from 2-4 carbon atoms.
 7. A coating system as recited in claim 6 in which said hydroxy functional monomer is hydroxy ethyl acrylate.
 8. A coating system as recited in claim 1 in which said styrene is present in an amount of from 65-80%, and the balance of said copolymer consists of adducts of an alkylene oxide containing 2-4 carbon atoms with an alpha, beta-monoethylenically unsaturated carboxylic acid.
 9. A low temperature curing polyurethane coating system comprising a mixture of a solution copolymer of from 65-80% of styrene with the balance of the copolymer consisting of monoethylenically unsaturated hydroxy functional monomer which is an adduct of an alkylene oxide containing 2 - 4 carbon atoms with an alpha,beta-monoethylenically unsaturated carboxylic acid, said copolymer being in solution in inert water-free organic solvent, and an organic polyisocyanate which is an aliphatic polyisocyanate having an isocyanate functionality of at least 3, said polyisocyanate being present in a stoichiometric excess of 5-40% with respect to the hydroxy functionality in said copolymer. 